1. Field of Use
This invention relates to a continuous process for preparing a cyclic acetal from a 1,3-diol and acrolein with substantially complete conversion of the acrolein. More specifically, this invention relates to a continuous process for the preparation of a cyclic acetal which comprises reacting acrolein and a 1,3-diol in the presence of a soluble acid catalyst and simultaneously separating by extraction therefrom the product cyclic acetal and water.
2. Prior Art
The reactions of alcohols with aldehydes to form acetals are equilibrium reactions. The degree of conversion to the acetal is limited by the equilibrium constant for the reaction, ##EQU1## UNLESS ONE OF THE PRODUCTS CAN BE REMOVED FROM THE REACTION SITE.
In the above equation, K is the equilibrium constant, and the various C's represent the molar concentrations of reactants and products. The equilibrium constant for the reaction of methanol with acetaldehyde allows only about a 50% conversion to acetal based on the aldehyde, while the reaction of acrolein with 2-methyl-1,3-propanediol (MPD) yields only 65% of acetal based on acrolein under equilibrium conditions.
Several techniques have been used in an attempt to obtain acetals at concentrations higher than the equilibrium concentration. The most common technique used is the completion of the reaction by azeotropic distillation of water with a water-insoluble organic solvent such as benzene or toluene as disclosed in U.S. Pat. No. 2,987,524. Such a process suffers from several basic deficiencies. The high volatility of the acrolein results in excessive quantities coming overhead, which must be separated from the water and returned to the reactor. The overall efficiency is low because low yields of acetal are obtained per volume of reactor space. A high energy consumption, because of the relatively high temperatures required for azeotropic removal of water, is needed. At the temperatures and times required for azeotropic distillation, unsaturated aldehydes, such as acrolein, react to form side products by polymerization and addition of water and alcohols to the carbon-carbon double bond.
Large excesses of one reactant, usually the alcohol, have been used to drive the equilibrium toward higher conversions with more complete utilization of the aldehyde. U.S. Pat. No. 2,566,559 teaches the preferred use of 4 to 5 moles of alcohol per mole of aldehyde. The acetal product must be separated from the large excess of alcohol, and the alcohol recovered and recycled to the process. High molar ratios of aldehyde to alcohol may also be used, but side polymerization reactions and additions to the double bond may consume some of the unsaturated aldehydes.
U.S. Pat. No. 4,024,159 discloses reactions of alcohols or diols with aldehydes that involve formation of liquid acetals wherein greater than equilibrium amounts of acetal are prepared by removal of the organic phase from the aqueous phase.
Thus it is known to react acrolein with a 1,3-diol to produce an essentially equilibrated mixture of reactants and products. However, such processes either involve charging of greater than stoichiometric quantities of reactants to drive the equilibrium reaction to the right or involve the separation of acetal product by distillation or decantation.
There is a need for a process that substantially eliminates the handling of unreacted acrolein from the aforesaid equilibrium reaction while at the same time reduces the energy consumption required for recovery of product by distillation and achieves substantially higher conversions of acrolein to cyclic acetal.